1. Field of the Invention
The present invention relates to an inverter system for driving an induction motor, and more particularly, to an inverter system with an improved control of an induction motor at the time of a power failure and restoration.
2. Description of the Related Art
In a motor system drive-controlled by an inverter system, generally, the motor has the characteristic of a rotating load, and thus a mechanical system of the motor stores an inertial energy. The inertial energy E that the mechanical system has is as in [Formula 1]:
                    E        =                                            1              2                        ⁢            J                    ⋆                                    ω                                                          2                        ⁡                          [              J              ]                                                          [                  Formula          ⁢                                          ⁢          1                ]            
Wherein ω represents the rotation angle speed of a rotating body, and J represents an inertial moment.
The construction of a general inverter system for driving an induction motor will now be described with reference to FIG. 1.
As depicted in FIG. 1, an induction motor system that applies a general inverter system comprises: an inverter system 10; an induction motor IM drive-controlled upon receipt of power from the inverter system 10; and a load 13 connected to the induction motor IM and driven by the induction motor IM.
The inverter system 10 converts a commercial three-phase AC voltage into a three-phase DC voltage by rectification and smoothing, and inverts this three-phase DC voltage into a three-phase AC voltage of variable frequency and variable voltage to provide it to the induction motor IM.
Then, the induction motor IM produces a torque of such magnitude as to be obtained in [Formula 2], and operates the load 13 by the produced torque.
                              T          m                =                              J            ⁢                                          ⅆ                ω                                            ⅆ                t                                              +                      B            ⁢                                                  ⁢            ω                    +                                    T              L                        ⁡                          [              Nm              ]                                                          [                  Formula          ⁢                                          ⁢          2                ]            
Wherein J represents an inertial moment, B represents a frictional coefficient, ω represents the rotation angle speed of a rotating body, and TL represents a load torque.
FIG. 2 is a block diagram showing a detailed construction of an inverter system according to the related art.
As depicted in FIG. 2, the inverter system according to the related art comprises: a rectifying circuit 23 for rectifying a commercial three-phase power supply 21; a smoothing circuit 25 for smoothing the pulse of the rectified DC voltage received from the rectifying circuit 23 and producing it into a direct current having a constant magnitude to provide it; an inverter 11 for receiving the DC voltage outputted from the smoothing circuit 25; a voltage detector 27 for detecting a DC link voltage from the smoothing circuit 25; and a frequency generator 29 for generating a voltage pulse signal having a target frequency according to the detected DC link voltage and outputting the generated voltage pulse signal having the frequency into the inverter 11.
Here, a DC voltage produced by rectifying and smoothing a three-phase AC voltage by a rectifying circuit and a smoothing circuit 25 is referred to as a DC link voltage.
The voltage detector 27 detects a DC link voltage from the smoothing circuit 25. And, the voltage detector 27 judges whether the detected DC link voltage is a preset low voltage. Then, the voltage detector 27 informs the frequency generator 29 of the judgment result. Here, the magnitude of the low voltage is set differently according to an input voltage of the inverter. For example, if the input voltage of the inverter is 220V, the low voltage is about 220V, and if the input voltage of the inverter is 440V, the low voltage is about 400V.
If the detected DC link voltage is not a low voltage, the frequency generator 29 outputs a voltage pulse signal of variable frequency and variable voltage according to a target speed of the motor IM into the inverter 11.
At this time, the inverter 11 is switched by the voltage pulse signal of variable frequency and variable voltage from the frequency generator 29, thus inverting the DC voltage outputted from the smoothing circuit 25 into a three-phase AC voltage and applying it to the induction motor IM.
On the other hand, if the detected DC link voltage is a low voltage (i.e., electrostatic voltage), the frequency generator 29 does not generate a voltage pulse signal of variable frequency and variable voltage according to a target speed anymore, and thus the inverter 11 is not able to apply a driving power to the induction motor IM.
If the detected DC link voltage is a low voltage (i.e., electrostatic voltage)<the induction motor IM produces a torque calculated by the torque balance equation of [Formula 2]. Because the induction motor IM has an inertial energy though it is not applied with a driving power. Therefore, the induction motor IM rotates for a predetermined time, i.e., a time for consuming the whole inertial energy with a frictional load and then stops even in a power-off state.
Meanwhile, in the case that the inverter system is normally operated, the frequency of the output voltage pulse signal of the frequency generator 29 gradually decreases and thus the speed of the induction motor IM becomes ‘0’.
However, in the event of an unexpected power failure, the inverter 11 does not output a driving power to the induction motor IM anymore, and the induction motor IM rotates by an inertial energy for a predetermined time and then stops.
That is, when the DC link voltage becomes below a low voltage due to a power failure, the power inputted into the inverter is cut off, and accordingly, the driving power of the induction motor is cut off. Here, the time for cutting off the output (driving power) of the inverter 11 after the DC link voltage becomes below a low voltage is determined by the amount of load and the capacitance of the capacitor of the smoothing circuit 25. For instance, in case of a constant torque load with respect to rated load, the cut-off time is 16 milliseconds, and in case of a variable torque load with respect to rated load, the cut-off time is 8 milliseconds.
However, the inverter system according to the related art has no means for storing a voltage value of the output voltage pulse signal of the frequency generator 29 finally outputted at the time of power failure. Therefore, at the time of power restoration after a temporary power failure of about 8 mSecs or 16 mSecs, the control of the inverter according to a target frequency is restarted from a start control, while the induction motor IM cannot be smoothly controlled because it rotates at a speed higher than a predetermined speed by an inertial energy.
Furthermore, in the induction motor system provided with an inverter control device according to the related art, the induction motor continues to rotate for a predetermined time even if the output of the inverter is cut off due to power failure, which may cause a problem to the safety of a user when the user accesses the induction motor system only with the perception of the inverter state (e.g., whether the inverter outputs or not).